On April 26, 2009, about 0415 eastern daylight time, a Douglas DC3, N136FS, operated by Four Star Aviation Inc., was substantially damaged after experiencing a cockpit fire during taxi at Luis Munoz Marin International Airport (TJSJ), San Juan, Puerto Rico. The certificated airline transport pilot, certificated commercial pilot, and two crewmembers were not injured. Visual meteorological conditions prevailed for the planned flight to Cyril E King Airport (TIST), Charlotte Amalie, United States Virgin Islands. A company flight plan was filed for the cargo flight conducted under the provisions of 14 Code of Federal Regulations Part 135.

The flightcrew reported that they were transporting mail. While taxing on "juliet" taxiway to runway 10, the instrument panel and cockpit floor erupted in flames while smoke filled the cockpit. As the pilots were shutting down the engines, they became overwhelmed with fire and smoke, and quickly exited the airplane along with the two cargo handlers. The flightcrew added that they did not note any abnormal instrument readings or any anomalies during preflight inspection.

The low-wing, tailwheel retractable-gear airplane, serial number 22405, was manufactured in 1942. It was powered by two Pratt and Whitney R-1830-90D, 1,200-horsepower engines. At the time of the accident, it was equipped with five seats due to a cargo configuration. The airplane had accumulated 19,952 total hours of operation at the time of the accident.

Examination of the wreckage by Federal Aviation Administration (FAA) inspectors revealed that the fire was intense, from the bulkhead behind the pilots’ seats, to the front of the airplane. Everything was melted in that area, including instruments, switches, tubing, hoses, and skin. The inspectors noted more damage on the right side of the fuselage. They also noted that the left propeller was partially feathered.

The inspectors observed that the majority of the wires, contained inside the main junction box, had very little damage, except for two wires that had insulation missing. The damage appeared to be associated with the routing of the two wires. Both wires were connected to the battery relay and they ran through wires in and around the exposed terminal studs. Heat damage was noted on the insulation of wires and other components that were in contact with the exposed wires. The wires ran from the battery relay to the forward section of the cockpit, where the fire started. Due to the fire damage, the inspectors were unable to determine what system the wires were associated with; however, one of the systems connected to the battery relay is the propeller feathering system.

The inspectors added that the fuel pressure was a direct indicating system. Fuel traveled directly to the instruments via rigid aluminum lines routed on the right lower side of the fuselage. Review of maintenance records by the FAA inspectors revealed that the airplane was maintained under an FAA Approved Airplane Inspection Program. The review did not reveal any evidence of the fuel pressure indicating system lines and hoses having ever been replaced; however, they were only required to be replaced on an as needed basis.

Further review of maintenance records revealed that the electrical system, instrument lines, and hoses through the nose compartment were required to be inspected on a Phase D inspection. The airplane's last Phase D inspection was completed on July 14, 2008. The airplane had accrued 313.1 hours of operation, from the time of the last Phase D inspection, until the fire. The airplane's most recent inspection was a Phase B inspection, which was completed on March 22, 2009. The airplane had accumulated 40 hours of operation since that inspection.

Review of the National Transportation Safety Board accident/incident database, and FAA service difficulty reports, did not reveal any similar events during the 5-year period preceding the accident.

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